1. Field of the Invention
The present invention relates to a solid-state imaging device and its production method.
2. Description of the Related Art
A solid-state imaging device of CCD (Charge Coupled Device) type has charge transfer electrodes to transfer signal charges from photoelectric conversion elements. These electrodes are juxtaposed to each other on a charge transfer channel formed on a semiconductor substrate so that they are driven sequentially.
An existing solid-state imaging device of CCD type is shown in FIG. 9 which is a schematic plan view.
It is composed of a semiconductor substrate 111 and photoelectric conversion elements 112, such as photodiodes, formed thereon which convert incident light into signal charges. Adjacent to these photoelectric conversion elements 112 is a vertical transfer element 113 which receives signal charges from the photoelectric conversion elements 112 and transfers them in the vertical direction. At the output end of each vertical transfer element 113 is the horizontal transfer element 114 which receives signal charges from each vertical transfer element 113 and transfers them in the horizontal direction. In addition, to the output end of the horizontal transfer element 114 is attached the output element 115 which converts signal charges transferred from the horizontal transfer element 114 into voltage for output.
A solid-state imaging device of CCD type 101 mentioned above needs improvement for a higher transfer rate. This need is achieved by reducing the resistance of the transfer electrodes 131 and 132 for the vertical transfer element 113 and the horizontal transfer element 114, respectively. Moreover, if the transfer electrodes 131 are made of an opaque metallic material, they have a decreased resistance and do not need to be covered with a shading film, which leads to an expansion of the photoelectric conversion area. As a result, the photoelectric conversion elements 112 can capture more light, which leads to an improved sensitivity.
The transfer electrodes of metallic material can be obtained in the following way. First, polysilicon electrodes are formed and an insulating material is applied onto them to fill their gaps and cover their tops. The insulating material covering the tops of the polysilicon electrodes is removed so that they are exposed, and then the exposed polysilicon electrodes are selectively removed. The hollows left after this step are filled with a metallic material. The excess portion of the metallic material that covers the insulating material is removed by chemical-mechanical polishing. The method of forming metallic electrodes as mentioned above is disclosed in Japanese Patent Laid-open No. 2007-12677.
The gap width between the transfer electrodes mentioned above is an important factor that determines the transfer efficiency of signal charges. It is known that the narrower the gap width, the better the transfer efficiency. Thus the horizontal transfer element has a very high density of electrodes, and this results in a large difference in pattern areal ratio between the imaging region and the transfer element.
The trend in the field of CCD is toward replacement of existing polysilicon with metal, such as tungsten, as a material of transfer electrodes (or toward the use of metal electrodes) for improvement in signal transfer rate and sensitivity, as mentioned above. The disadvantage of metal electrodes is that there is a difference in areal ratio between the transfer electrodes 131 of the vertical transfer elements 113 (which are arranged in the pixel region) and the transfer electrodes 132 of the horizontal transfer element 114 (which is arranged in the periphery of the pixel region), as shown in FIG. 10A. Incidentally, the term “areal ratio” represents the ratio (in %) of the area of the electrode that accounts for in one pitch of the transfer electrodes 131 of the vertical transfer elements 113 or the transfer electrodes 132 of the horizontal transfer element 114.
The areal ratio of the pattern affects the polishing characteristics when an excess portion of the metallic material 151 is removed from the insulating material 141 by chemical-mechanical polishing as shown in FIG. 10B. The result is erosion that occurs on the surface of the transfer electrode 132 which has a large areal ratio, as shown in FIG. 10C. This erosion results in a step which is larger in the surface of the transfer electrode 132 of the horizontal transfer element than in the surface of the transfer electrode 131 of the vertical transfer element.